1. Field of the Invention
The present invention relates to an optical device and device manufacturing method.
2. Description of the Related Art
An EUV exposure apparatus using EUV light is known. A plasma light source can be used as an EUV light source for generating EUV light. Examples of the method of the EUV light source are the laser produced plasma (LPP) method and discharge produced plasma (DPP) method. In the laser produced plasma method, for example, a high-temperature plasma is generated by irradiating a target material placed in a vacuum vessel with high-intensity pulsed laser light, and the generated plasma is used as a light source. The plasma emits EUV light having a wavelength of, for example, about 13.5 nm. A metal, inert gas, or the like is used as the target material. To increase the average intensity of the EUV light emitted from the target, the repetition frequency of the pulsed laser is desirably high, and the operation is normally performed at a repetition frequency of a few kHz. A condenser mirror is used to efficiently utilize the EUV light emitted from the target. As the condenser mirror, it is possible to use, for example, a multilayer mirror obtained by alternately stacking about 60 molybdenum and silicon films, or an oblique incidence mirror coated with a metal.
An illumination optical system of the EUV exposure apparatus includes, for example, a plurality of multilayer mirrors and an optical integrator. The optical integrator has a function to uniformly illuminate a reticle (original) with a predetermined numerical aperture. Collimated EUV light enters the optical integrator and generates a secondary light source at the focal point.
The EUV light supplied from the illumination optical system is reflected by the reticle, and reduced to ¼ by a projection optical system including, for example, six to eight multilayer mirrors. The reduced EUV light irradiates a wafer (substrate) coated with a resist. The reticle and wafer are respectively held by a reticle stage and wafer stage, and synchronously scanned at a speed ratio proportional to the reduction magnification. In this manner, the operation of synchronously scanning the reticle and wafer while the reduced projected image of the reticle is formed on the wafer is repeated (step-and-scan). Thus, the reticle pattern is transferred onto the entire surface of the wafer.
The EUV light source generates not only the EUV light having the desired wavelength, but also light having an unnecessary wavelength from the infrared region to the X-ray region. This light having the unnecessary wavelength is called OoB light (Out of Band light). The OoB light acts as flare and decreases the contrast of light on the wafer or thermally expands the wafer. Also, the OoB light raises the temperature of a mirror, and this poses a problem concerning a mirror of particularly a projection system. The LPP light source poses the problem of scattered light from a CO2 laser for use in plasma excitation. This scattered light contains infrared light having a wavelength of 10.6 μm, and this infrared light thermally expands the reticle or wafer, degrades the imaging performance, or decreases the overlay accuracy.
As a method of removing the OoB light in the infrared region, there is a method of incorporating a wavelength separation filter as an additional optical element in the light source or the illumination optical system of the exposure apparatus. There is also a method of separating light generated by a light source into EUV light and infrared light by inserting a reflection blazed grating as the wavelength separation filter in an optical path. The surface of the blazed grating is coated with a multilayer film for reflecting EUV light. The blazed grating causes diffraction in a direction corresponding to the wavelength of incident light. Therefore, the blazed grating can separate EUV light and infrared light by guiding them in different directions (Japanese Patent Laid-Open No. 2005-302998).
In the conventional apparatus, however, a light amount loss corresponding to the reflectance of the grating surface of the wavelength separation filter to the EUV light is produced because the wavelength separation filter is additionally installed in the light source or illumination optical system. Also, the OoB light reflected by the wavelength separation filter may scatter inside the exposure apparatus and warm mirrors and a chamber of the illumination system, a wafer, and the like. Accordingly, demands have arisen for an optical device that separates light having a first wavelength from light having a second wavelength and exerting different powers on these light components.